1. Field of the Invention
The present invention relates to an apparatus and a method of nondestructive inspection, which examines whether a shaft, beam or pipe has some cracks. More particularly, the present invention relates to the apparatus and the method for structural diagnosis, which generates torsional waves upon a shaft, beam or pipe having arbitrary cross-sections such as a circular cross-section, and senses existence or location of the cracks through analyzing reflected waves.
2. Background of the Related Art
Magnetostrictive effect, which is also called Joule effect, refers to a phenomenon that mechanical deformation occurs in a ferromagnetic material when the ferromagnetic material is placed in magnetic fields. A reciprocal phenomenon of the magnetostrictive effect (i.e. a phenomenon that changes in magnetic field in vicinity of materials occur when mechanical stress is applied to the materials) is designated as inverse magnetostrictive effect or Villari effect.
The magnetostrictive effect can be expressed as follows:B=μ0H+μ0M=μ0H+μ0χmH=μ0(1+χm)H=μ0μrH=μH  [Equation]
where B denotes magnetic flux density, H denotes magnetic field intensity applied by an outside magnet or electromagnet, M denotes magnetization, χm denotes magnetic susceptibility, μr denotes relative permeability, and μ denotes permeability.
The Joule and Villari effect can be expressed as equations 2 and 3:
                    ɛ        =                              σ                          E              H                                +                                    q              *                        ⁢            H                                              [                  Equation          ⁢                                          ⁢          2                ]                                B        =                                            μ              σ                        ⁢            H                    +                      q            ⁢                                                  ⁢            σ                                              [                  Equation          ⁢                                          ⁢          3                ]            
where ε denotes strain, σ denotes stress, EH denotes modulus of elasticity in a constant magnetic field, and μσ denotes permeability when constant stress is applied.
The coefficients related to the Joule effect and Villari effect can be represented as equation 4 and equation 5, respectively:
                                                        q              *                        =                                          ⅆ                ɛ                                            ⅆ                H                                              )                σ                            [                  Equation          ⁢                                          ⁢          4                ]                                                      q            =                                          ⅆ                B                                            ⅆ                σ                                              )                H                            [                  Equation          ⁢                                          ⁢          5                ]            
Magnetostrictive transducers can generate elastic waves of high energy density without physical contact with a test specimen. Therefore, it has been applied to various technical fields. Moreover, it can generate various types of elastic waves such as longitudinal, flexural, and torsional waves only with changes of bias magnetic field.
Among various types of waves, torsional waves are very attractive for the damage detection of the rod or pipe since a first mode of torsional waves has no dispersion. “Dispersion” refers to a phenomenon that waveform of propagating wave is distorted as velocities of waves depend on frequencies.
Nickel is soft magnetic material, that is, it has relatively low hysteresis and small residual flux density. Accordingly, it returns closely to its original status after removing the applied magnetic field. So nickel does not need de-magnetization for repeatability of the transducer, even though, in general, time history affects significantly state of magnetic material.
FIG. 1 is the group velocity dispersion curve of the torsional waves traveling along the aluminum pipe whose thickness(ts) is 1 mm, outer diameter(d0) is 25 mm, density(ρ) is 2800 kg/M3 and modulus of elasticity (E) is 73 GPa.
As can be seen in FIG. 1, a first mode of torsional waves is non-dispersive; therefore, the waveform does not change during its traveling. It is very useful for the long range inspection of a pipe since the traveling time can be measured accurately.
FIG. 2 is a drawing showing structure of the magnetostrictive transducer of the related art, which is used for actuating and measuring the torsional waves.
As illustrated in FIG. 2, the magnetostrictive transducer of the related art is comprised of a thin nickel strip 1 which is bonded around a rod member 2 in a circumferential direction, a non-ferromagnetic housing 4 which is needed to install coil 3.
For detection of cracks in a rod member 2 with the magnetostrictive transducer of the related art, the nickel strip 1 should be pre-magnetized by rubbing a permanent magnet in a circumferential direction (not illustrated). By adjusting the magnitudes of the axially applied magnetic field from the coil, the elastic waves are generated and propagate in the oblique direction, e.g. 45°, with axial direction if the magnitude of the axially applied magnetic field amounts to the magnitude of pre-magnetization in the strip 1. The generated torsional waves travel along the rod member 2 and are reflected from a part where cracks exist, if cracks exist. Then, the reflected torsional waves change the magnetic field around the strip 1.
The variance of the magnetic field induces the electromotive force at the coil by the Faraday-Lenz law, which is described in equation 6. Therefore, the elastic waves propagating along the pipe can be measured by picking out the voltage at the ends of the coil.
                              V          ⁡                      (            t            )                          =                              -            N                    ⁢                                    ⅆ              Φ                                      ⅆ              t                                                          [                  Equation          ⁢                                          ⁢          6                ]            
where φ denotes the total amount of magnetic flux, N denotes the wounded number of the coil, and t denotes the time.
FIG. 3a and FIG. 3b are graphs showing the measured signals of the elastic waves (especially, torsional wave) with the transducer of the related arts. Input currents are 3A in FIG. 3a and 6A in FIG. 3b. 
As illustrated in FIG. 3a and FIG. 3b, in the related art it was difficult to send torsional waves with large output power, because the wave pattern measured at the transducer is very sensitive to the input current. When the magnitude of the magnetic field by the input current is increased, the magnitude of the magnetic field by the pre-magnetization with a permanent magnet becomes relatively insufficient for torsional waves. Therefore, besides torsional waves, longitudinal waves are also generated, thus making the measured signal complex as shown in FIG. 3b. 
That is, in the related art, if the magnitude of the axially applied magnetic field from the coil and the quantity of pre-magnetization are not well determined, other kinds of waves, besides torsional wave, are also produced. Waveforms of other elastic waves except torsional wave can be considerably changed at the time of being reflected, so that it is not suitable for the other elastic waves to be used in detecting cracks. In addition, as pre-magnetization undergoes time-decay, nickel strip has to be pre-magnetized at every inspection.
Accordingly, for the effective detection of the cracks in the rod, beam or pipe members having arbitrary cross-sections, such as circular cross-section, it is requested that transducer and the diagnosis apparatus using the transducer, which can stably generate large magnitude of torsional wave and secure repeatability, be developed.
The present invention was developed to solve the problems mentioned above. The present invention is to provide an apparatus for structural diagnosis, which can stably generate large magnitude of torsional wave upon a shaft, beam or pipe member having arbitrary cross-sections, such as circular cross-section, without any special pre-magnetization process using a permanent magnet etc., and can detect existence or location of the cracks through analyzing reflected waves.